Ben’s death is truly one of the biggest mysteries in The Umbrella Academy.
The rest of the Hargreeves siblings acknowledge his death, but they never bring up exactly how he died.
Despite being dead, Ben was still a part of the Netflix series, primarily because Klaus’ powers allowed him to convene with the deceased.
Ben would offer sage advice occasionally or give his opinion about the bad ideas Klaus gets himself involved with, but none of his other siblings seemed to realize who Klaus was talking to.